I'd have to say comedies. They are probably the most comforting movies to watch; even if you're in a bad mood, pop in a comedy and it will lighten up your day. I think every other genre I have to be in a specific mood for, whereas with comedies it doesn't really matter. Unless I'm incredibly angry I can throw a comedy on and get absorbed into it, or not. You don't really have to give full attention to them because they don't really have complex plots most of the time- you can just sway in and out of paying attention and be amused by the jokes.